1. Field of the Invention
This invention relates to the field of communication control for interconnecting one or more digital devices to one or more other digital devices. More specifically, the invention concerns an apparatus for interconnecting a moderate to large number of user terminals, computer systems, remote digital devices, and the like, using some of the concepts inherent in statistical multiplexers, intelligent switches, and local area network devices, all implemented in a compact configuration to produce a flexible and cost effective approach to data communication interfaces. The communication control apparatus of this invention replaces and simplifies the actual terminal connections from the computer interface(s) and then adds substantial system enhancements for the manager while retaining software and protocol transparency. In this description, the term "digital device" will refer to any apparatus whose communication ports transmit and/or receive digital data, such as mainframe computers, mini computers, terminals, modems, printers, displays, and like devices with digitally formatted interfaces.
2. Description of the Prior Art
As telecommunications companies, computer firms, and end user requirements grow, the need for improved data communication within a computer system expands at the same or even greater rate. Many of the problems associated with prior art systems, as they are assembled or modified for expansion, result from the increased utilization of many computer networks, a significant shift to arrangements involving separate clusters of terminals, the need for placing large numbers of terminals at remote locations from the computer room, and broader usage of common carriers (i.e. the telephone companies) for data transmission.
It is common to find communication networks associated with the mainframe user composed of cluster controllers to which groups of terminals are connected, with the data being transferred from the controller to the control site via a high-speed composite line. Unfortunately, such prior art controllers were not available to minicomputer users whose systems, because of their development as on-line rather than batch processing systems, lacked the front-end processors, cluster controllers, and special communications protocols of their larger counterparts.
Typically, a system involving a minicomputer with a number of terminals distant from the control site would simply be interconnected using individual leased lines. Certain data concentrators, widely known as statistical multiplexers, have recently become available in an attempt to minimize the line costs involved in communicating along a multiline network between a computer interface and the cluster of user terminals. Such statistical multiplexers accommodate a large number of lines from the computer interface and communicate the information down a single high-speed composite line to be restored to a multiline arrangement at the other end. A statistical multiplexer, however, is merely a means for concentrating signal information down to a single high-speed transmission line between two points which may be spaced perhaps one kilometer apart. At each transmit/receive end of the high-speed composite line, complex and expensive signal handling apparatus is required to make the requisite conversions between multiline-to-single line and single line-to-multiline transformations. Unless a considerable number of lines are involved in the system, it is possible that the cost of statistical multiplexer devices would not be offset by the saving in line costs.
A mainframe user may also have a network to manage consisting of a large quantity of terminals and a number of host computers. Each user has a need to gain access to the different host computers via their own terminal. At the same time, there is a need in such systems for operations managers to control and manage the network. To meet these requirements, digital data switching systems have recently become commercially available. Such switching systems permit the interconnection of more terminals to the system than there are ports available on the computers, and to help manage the system the microprocessor-based design permits supervisory control, queueing, event logs, statistics, and the like. Such systems, unfortunately, are expensive, often costing more than the entire minicomputer system itself. Moreover, since all lines from all devices must route through the switcher, more often than not the number of lines in a system were greatly increased, not decreased, in favor of the improved communication control. Switching networks have other significant drawbacks: they nearly double the number of interconnects in a system, increasing the chances for mechanical breakdown; and they cannot establish two-way communication between all devices, resulting in primary devices being able to select any secondary device but with secondary devices being unable to select destinations. Such systems did, however, establish the basic guidelines for intelligent switching systems which have become more attractive as the price of minicomputers, particularly superminicomputers, rises. Still, below 64 lines of network control via an intelligent switch is a luxury most minicomputer users cannot afford.
Local area network devices, defined as intelligent switching systems which are able to be interconnected and placed remotely while being controlled from a single source, introduced the concepts of redundancy and distributed architecture. These concepts have been of substantial attraction in recent years to users of a minicomputer data communications system. Again, however, the cost of such devices, designed for general application, is prohibitive to the minicomputer user.
While multi-terminal systems have grown considerably in recent years, and although certain options are commercially available for enhancing such systems, such as the statistical multiplexers, intelligence switching systems, and local area networks discussed above, the basic minicomputer data communications multiplexer has undergone little change other than reduction of physical size and cost. Thus, there is clearly a need for an improved and cost effective computer/digital data communications multiplexer which can support a large number of terminal connections, which allows a terminal port connection to be placed at a long distance from a host computer adapter, that is expandable in function and in number of terminal connections, that may permit communication to more than one host computer, that permits intercommunication between any two or more digital devices, and that provides supervisor and printer port capability for permitting on-line configuration capability and a log of all connections. The present invention satisfies this need.